Preheat your oven to 220 degrees C / 425 degrees F / gas mark 7.
Rub some butter or oil on to a baking sheet.
Put the leeks in a lidded microwave dish.
Add enough water to JUST cover the bottom of the dish and a knob of butter about as big as a conker.
Put the lid on and cook in the microwave for about 4-5 minutes to soften them a bit.
Roll out your puff pastry to the size of your baking sheet and carefully move it on to it.
With a normal dinner knife, score a square around the pastry about 3cm in from the edge, so it’s like a picture frame.
Scoring means scratching a line into the pastry but not going all the way through.
Brush the ‘frame’ part with milk.
Pile the leeks into the middle of the frame and spread them around to fill the ‘picture’.
Put the tart in the oven for 15-20 minutes until it is puffed up and crispy.
While it’s in the oven, mix the yoghurt and mustard together so the flavours can mix.
When the tart comes out, serve it with a dollop of the mustardy yoghurt on top.

Tips:The pastry should be quite thin or it will be all soggy in the middle – thicker than a pound coin but not as thick as two pound coins.
Instead of the yoghurt topping, try it in a gravy dinner or with chutney and salad – yum!
Make this tart with different seasonal fillings. In spring how about purple sprouting broccoli (steamed in the same way) with goats cheese and chopped chilli? Or courgettes in summer, peppers in autumn?

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